High solids content solvent-borne coating compositions based on urethanes or on melamine-crosslinked resins have been used for demanding end-use applications such as coatings on farm implements and construction equipment. Especially durable coatings may be made by including in the coating composition a substantial proportion of a low molecular weight, low polydispersity acrylic polyol. A class of such acrylic polyols is sold under the JONCRYL™ name by BASF Performance Chemicals. They are believed to be made in a pressurized reactor system using a so-called “solid grade oligomer” process according to one or more of U.S. Pat. Nos. 4,414,370, 4,529,787 and 4,546,160.
When making polymers, the polymerization process may be limited by a number of factors including the reaction mixture viscosity and the viscosity buildup rate during polymerization. This can be a particular concern when making high solids, low solvent content polymers, as it may be difficult to stir the polymer adequately. The reaction mixture viscosity can be lowered and stirring can be facilitated by carrying out polymerization in a high boiling point solvent. However, high boiling point solvents also tend to have slow evaporation rates and may be difficult to remove by vacuum stripping. These and other obstacles have made it difficult to form copolymers having low molecular weight and low polydispersity, especially when it is desired to avoid the high capital expense associated with the use of a pressurized reactor system.